1. Field of the Invention
The present invention relates to a polymer secondary cell electrode production method and in particular, to a polymer secondary cell electrode production method using a high molecular material exhibiting an electro-chemical oxidation-reduction reaction and an conductivity assisting agent.
2. Description of the Related Art
Recently, the environment and energy problems increase the interest for electric vehicles (EV) or hybrid cars. Conventionally, the most important problem for the electric vehicles and the hybrid cars has been associated with a battery element such as a cell capacitor. The battery element has technical problems of (1) low energy density, (2) low power density, and (3) low cycle characteristic.
In order to solve the problem of the low energy density, it is necessary (1) to increase the cell electromotive force, (2) to increase the capacity of the electrode active material, (3) to improve the cell volumetric efficiency, and the like. The solutions (1) and (2) are very difficult because they require a development of a new active material and a development of an electrolyte having a wide potential window. On the other hand, the solution (3) can be realized by increasing the electrode film thickness to increase the film thickness ratio of the electrode/collector, thus improving the energy density.
According to the conventional electrode production method, an electrode active material, binder, conductivity assisting agent, and solvent are mixed into a slurry, which is applied to a collector using a doctor blade or the like. However, the electrode film thus prepared has a thickness not greater than 200 micrometers and the film thickness ratio of the electrode against the collector is as low as 10. This is because the binder is solved in a solvent and the binder shrinks when dried, which in turn brings shrinkage of the electrode itself. This causes cracks and breakage of the electrode and peeling off of the electrode from the collector. This phenomenon becomes more remarkable as the electrode film thickness becomes thicker. Moreover, if the binder content is increased to prevent cracks of the electrode, there arises a problem that the binder molecules cover the electrode active material, lowering the cell performance.
In order to solve this problem, Japanese Patent Publication 8-64200 [1] discloses an electrode made from a high molecular material exhibiting an electro-chemical oxidation-reduction added by a plasticizer such as phthalic acid. This electrode increases the flexibility by adding the plasticizer to the high molecular material.
Moreover, Japanese Patent Publication 61-214418 [2] discloses an electrode prepared from activated carbon fiber which is formed into a paper shape using a thermal fusion binder medium such as polyethylene and polypropylene and subjected to a thermal press or thermal calendar processing so that the activated carbon fiber and the binder medium are thermally fused.
However, the aforementioned conventional techniques have problems as follows. In the case of the electrode disclosed in document [1], although the plasticizer increases the flexibility of the electrode, since film formation is performed by using the high molecular material exhibiting an electro-chemical oxidation-reduction and the plasticizer dissolved in a solvent, the high molecular material shrinks when dried and cracks are easily caused in the electrode when a thick film electrode is formed even when the plasticizer is added. Accordingly, the electrode film thickness should be limited to a range of 20 to 100 micrometers.
Moreover, in the case of the electrode disclosed in document [2], a high molecular binder such as polyethylene and polypropylene not exhibiting oxidation-reduction is used and the binder molecule covers the electrode active material, lowering the cell performance.
It is therefore an object of the present invention to provide a polymer secondary cell electrode production method using a polymer active material exhibiting an electro-chemical oxidation-reduction and a conductivity assisting agent which solves the aforementioned conventional technical problems.
The polymer secondary cell electrode production method according to the present invention comprises steps of: mixing a polymer active material powder exhibiting an electro-chemical oxidation-reduction reaction and a conductivity assisting agent powder; and molding the mixture by thermal press into a predetermined thickness.
According to another aspect of the present invention, the polymer secondary cell electrode production method comprises steps of: coating a conductivity assisting agent with a polymer active material exhibiting a electro-chemical oxidation-reduction reaction into a coated powder; and molding the coated powder by a thermal press into a predetermined thickness.
The polymer active material may be selected from a group consisiting of polyaniline, polypyrol, polythiophen, polyacetylene, polyvinyl carbazole, polytriphenylamine, polypyridine, polyopyrimidine, polyquinoxaline, polyphenylquinoxaline, polyisothianaphten, polypyridinezeal, polythienylene, polyparapffinylene, polyfluran, polyacen, polyfuran, polyazulene, polyindol, and polydiaminoantraquinon.
The temperature of the thermal press should be not lower than the glass transition point or the melting temperature of the polymer active material. At this temperature, the polymer active material is made into a rubber state partially or melted. When the polymer active material in this state is pressed, particles of the material adhere to each other or particle and the conductivity assisting agent adhere to each other. Thus, the melted particles adhere to one another and the electrode is molded into a unitary block. When this electrode is cooled, the shrinkage is small because the particles are melted only partially. Accordingly, cracks will not be caused easily when the electrode film is made thick.
By performing the thermal press in a nitrogen gas atmosphere, it is possible to suppress oxidation of the polymer active material, thus enabling to maintain the oxidation-reduction activity of the polymer active material.
The conductivity assisting agent powder may be one or more than one in combination selected from a group consisting of acetylene black, Ketjen black, epitaxial carbon, graphite powder, aniline black, activated carbon powder and other conductive carbon powder, polyacrylonitirile, pitch, cellulose, phenol resin, or sintered carbon powder formed from palm shells, oxide powder of Ti, Sn, or In, metal powder such as stainless steel, Ni, Au, Ag, Ta, Nb, Cu, and Al.